1. Field of the Invention
The present invention relates to a method of preparing a substrate having a layer or pattern of metal on it, to a substrate prepared by such method and to uses thereof.
2. Description of the Related Art
During the past decade, soft lithography has developed to a versatile technique for fabricating chemically micro and nano-structured surfaces. Among several techniques known collectively as soft lithography, micro contact printing (μCP) has become the most commonly used method. The technique was initially developed for the transfer of molecules and was also applied for the transfer of metals.
Two soft-lithographic methods for contacting organic materials with metals have been developed up to now, namely nano-transfer printing (nTP) and soft-contact lamination (ScL). They can be used for the parallel fabrication of multiple devices. Both methods are schematically depicted in FIG. 1.
In case of nTP in FIG. 1a, a thin layer of metal is evaporated onto a patterned elastomeric stamp, which has been fabricated by drop casting of polydimethylsiloxane (PDMS) onto a patterned Si wafer. The evaporated metal layer is brought into conformal contact with an organic layer on a substrate. As a result of the chemical bond formation at the metal-organic interface, the metal-organic adhesion is stronger than the metal-PDMS adhesion and the metal layer is transferred from the PDMS stamp onto the organic layer.
The nTP process takes place under ambient conditions without application of any additional pressure. This process has been demonstrated by the fabrication of Au top electrodes in Au/alkanedithiol/GaAs hetero junctions and Au/mercaptosilane/Si hetero junctions. In another process gold was patterned on Silicon wafers and subsequently transferred to selected polymers at high pressure (9-30 bar) and temperature between 100 and 140° C.
In case of ScL, the metal-organic adhesion is based on van der Waals interactions and is weaker than the metal-PDMS interaction. Thus, in this process, the metal is not transferred from the PDMS onto the organic layer, but the PDMS remains on the Au layer and is part of the PDMS/metal/organic/substrate hetero junction (FIG. 1b). The metal layer is prepared on top of an unstructured flat PDMS layer using shadow mask evaporation. The process takes place under ambient conditions without application of any additional pressure.
Furthermore, the present inventors have developed another process, called Shuttle Transfer Printing (STP) which is disclosed in European Patent Application No. 06006899.6, filed on Mar. 31, 2006 which can be scaled down to dimensions as low as 50-100 nm. This fabrication process is easy to perform and versatile with respect to different metals, metal oxides, and semiconductor materials.
Usually, in the aforementioned techniques, the layer or pattern of material that is transferred from one substrate to a target substrate has a certain roughness itself which roughness may interfere with the transfer and subsequent adhesion process. The roughness associated with the layer or pattern of material to be transferred, in many instances prevents a good contact between the target substrate and the pattern of material, and hence, the transfer may be inefficient or incomplete.